228 BLIND VERTEBRATES AND THEER EYES. 



By the time the young are well advanced, i.e., 18 to 20 mm. long, the lymph 

 sinuses of the stroma have mostly lost their contents and the stroma itself has 

 become very greatly reduced and compressed into a narrow median wall (plate 29, 

 fig. b). There are no "pockets" in which the young are carried as in Cymatogaster 

 and other Embiotocida (Eigenmann), or as in Anablcps (Wyman), but instead the 

 young attach themselves by the mouth to the ovarian lobes, or lie free within the 

 lumen of the ovisac. 



The single oviduct, as well as the ovisac, is widely distended in pregnant females 

 when the young are well advanced. In the non-pregnant females the duct is a 

 cylindrical, thick-walled, muscular tube with numerous folds on its inner surface, 

 which is covered with a layer of columnar epithelium similar in all respects to that 

 of the ovisac. 



MINUTE STRUCTURE. 



The Ovisac. — The ovisac, as noted, varies greatly in appearance, depending 

 on the length of the pregnancy or the time since the close of that period. In nor- 

 mal, non-pregnant ovaries it varies from 100 to 150 jx in thickness. Structurally 

 it is composed of at least 4 cell layers. The outermost is the ordinary peritoneal 

 layer continuous with the lining of the body cavity; second, a thicker layer of 

 longitudinal muscle fibers which lie immediately below the peritoneal covering; 

 third, a somewhat thicker band of transverse muscle fibers ; fourth, the inner lining 

 of epithelium, which, in some instances at least, contains numerous blood capillaries. 

 In the case of pregnant females, the ovisac is more or less thinned through 

 stretching, until, when the young are well advanced, the cell layers can scarcely be 

 distinguished. (See plate 29, figs. A, b, c.) 



The Ovary. — The ovarian structures proper are highly vascular and much lobed. 

 In some instances the egg follicles are surrounded by a network of blood capillaries. 

 The greater part of the stroma is split up into numerous sinuses (st., plate 28), 

 many of which are larger than any of its blood-vessels. The epithelial layer cover- 

 ing the stroma frequently contains numerous capillaries each with a diameter of 



5 to 8 fi (plate 29, fig. f). In some instances these capillaries are very numerous ; 

 in others, they are scarcely, if at all, perceptible. This difference is due to the 

 different degrees of distention of the stroma by the lymph in its sinuses. The 

 stroma itself consists of a mass of connective tissue and non-striated muscle fibers 

 in which are embedded the ova in various stages of development. 



The Follicle. — The smallest ova (5 to 10 fi. in diameter) have no trace of a follic- 

 ular membrane around them individually. Somewhat larger ova (100 to nearly 

 400 fi in diameter) are surrounded by a single layer of elongate cells, quite similar 

 to the stroma cells. In the case of more mature ova (over 400 fx in diameter) there is 

 a distinct follicle consisting of a single layer of appressed quadrangular cells about 



6 /x in depth ; outside of this is a layer of somewhat irregular cells, in many cases 

 surrounding blood capillaries 6 to 10 \x in diameter. The thecal wall outside the 

 capillary layer consists of from 1 to 3 cell layers of long, spindle-shaped cells 

 resembling those composing the stroma itself. The medium-sized ova (about 

 400 /x in diameter) lie close beneath the surface of the ovary (0., plate 29, fig. d), but 

 the largest ova (600 jx and over in diameter) are usually found rather deeply em- 

 bedded within the stroma, except for the tubular indentation from the surface of 



